The invention relates to a frequency synthesizer comprising a voltage-controlled oscillator, a divider for dividing the frequency of said oscillator, a display unit for displaying the desired frequency from which there is subtracted a digital control data of the dividing ratio of the frequency divider, a phase-locked loop comprising a phase comparator which receives the divided oscillator frequency and a reference frequency and supplies a frequency control voltage for the oscillator, a search tuning loop comprising a frequency discriminator which receives the divided frequency and the reference frequency and which supplies pulses when the frequencies are different, these pulses being counted in a counter which is automatically reset to an initial position when it reaches an end position, and converting means for converting the digital data constituted by the content of said counter into a search tuning voltage which is used to control the frequency of said oscillator.
A frequency synthesizer of the type comprising a frequency divider and a phase-locked loop and a search tuning loop is disclosed in, for example, French Patent Specification No. 2.170.908, to which U.S. Pat. No. 3,825,855 corresponds. The search tuning loop supplies, when the displayed frequency is changed, a step-wise variable search tuning voltage to have the natural frequency of the oscillator vary automatically and to adjust it to the narrow range where the phase-locked loop can become operative and lock the phase of the oscillator frequency onto a multiple of the reference frequency or a fraction of this frequency. But it will be clear that the search tuning frequency from which the desired frequency is obtained may be rather long, particularly when the frequency synthesizer must cover a large range of frequencies, whereas the automatic search tuning voltage, whose rate of variation is considerably limited, must pass through a wide range corresponding to the frequency range of the synthesizer.